The present disclosure relates generally to aircraft braking systems and, more particularly, to brake selection systems and methods for reducing carbon brake wear.
Modern aircraft which are designed to carry large passenger or cargo payloads are often provided with carbon brakes on each of the wing or body mounted wheels. While carbon brakes are preferred over steel brakes for weight and performance reasons, the cost of replacing the stack divided by the number of landing cycles between replacements can be much higher than for steel brakes. Further, carbon brakes primarily wear based on the number of applications (including taxi braking), whereas steel brakes primarily wear based on the amount of heat energy generated. Therefore, steel brake wear is less sensitive to the number of taxi brake applications.
At least one known braking method includes evenly dividing the braking energy between all of the brakes by activating all the brakes equally. By activating all brakes equally, no individual brake absorbs an excess of energy. For steel brakes, brake life is largely determined by the total amount of energy absorbed by each brake and is comparatively unaffected by the number of brake applications that accumulate that energy. Hence, brake control systems that activate all brakes simultaneously and equally provide economic operation of steel brakes and reduce exposure to overheating of any individual brake. In contrast to steel brake wear, carbon brake wear has been found to correlate significantly with the number of brake applications. Specifically, most carbon brake wear tends to occur during taxiing, as the brakes may be activated routinely in negotiating the taxiways between the runway and the gate and in stop-and-go traffic that may be encountered in the queue for take-off. As such, application of the steel braking method to carbon brakes may significantly shorten the operating lifetime of carbon brakes.
An additional important factor in the wear rate of carbon brakes is the brake core temperature. Depending on the wear state and the unique and transient friction characteristics of individual brakes, there is typically significant brake core temperature variance between brake positions on an aircraft such that the temperature varies brake-to-brake. Variance in brake cooling rate can exacerbate these brake temperature differences depending, for example, on the relative proximity to air flow.
Another known braking method includes only activating as many brakes that are necessary for that particular taxi braking event. Such systems may sequentially cycle through the brakes such that each brake is activated only a minimum number of times. However, as described above, some brakes will have higher core temperatures than others because some brakes absorb more heat energy from the same braking event than other brakes. Therefore, by merely cycling through the brakes for subsequent taxi braking events, some brakes may still be at a relatively high temperature, which may lead to increased brake wear or oxidation and limit the service lifetime of the brake.
Apart from brake wear, there is the consideration of the issue of carbon oxidation of very high temperature carbon. Carbon reacts with oxygen at high temperatures to gradually form oxides, which may limit the service lifetime of the brakes. During higher landing energy operations, there is often sufficient disparity between individual brake temperatures on an airplane such that some brake positions enter the high oxidation rate temperature range, whereas other brakes may be below the high oxidation rate threshold to allow additional taxi stops while still remaining below the threshold temperature.
Accordingly, there is a need for a brake selection system that limits the number of applications of each brake and that also monitors the brake temperature to reduce brake wear and oxidation.